Wireless communication systems can be largely divided into a frequency division duplex (FDD) scheme and a time division duplex (TDD) scheme. Uplink transmission and downlink transmission occupy frequency bands different from each other in the FDD scheme, and uplink transmission and downlink transmission occupy the same frequency band in different time in the TDD scheme. In the TDD scheme, channel response is mutual dependent. That is, a downlink channel response is the same as an uplink channel response in a given time-frequency. Accordingly, a wireless communication system of a TDD scheme is advantageous in that the downlink channel response can be obtained from the uplink channel response.
According to a hybrid automatic repeat request (HARQ) scheme, performance is enhanced by confirming whether data received by the physical layer contains an error that cannot be decoded and requesting retransmission if there is an error. If no error is detected in the received data, the receiver transmits an ACK signal as a response signal to inform the transmitter of success of receiving the data. If an error is detected in the received data, the receiver transmits a NACK signal as a response signal to inform the transmitter of detection of the error. If the NACK signal is received, the transmitter may retransmit the data.
In a TDD system, one radio frame includes at least one downlink subframe and at least one uplink subframe. An ACK/NACK signal, which is a response to downlink data transmission, should be transmitted for each downlink subframe. Since the ACK/NACK signals need high reliability, it is general that the number of ACK/NACK signals that can be transmitted on one control channel is limited. Downlink channel needs to be scheduled in accordance with the ACK/NACK signal.
There needs a method that can efficiently schedule downlink data within limited radio resources in a TDD system.